Silicon carbide (SiC), of which the band gap and the dielectric breakdown voltage strength are greater than those of silicon (Si), is a semiconductor material which is expected to be applied to next-generation low-loss power devices, for example. SiC exists in various polytypes such as 3C-SiC which is a cubic system, 6H-SiC and 4H-SiC which are hexagonal systems. Among the polytypes, 4H-SiC is generally used for producing silicon carbide semiconductor devices.
A typical power device including SiC and serving as a switching element is a field effect transistor such as a metal insulator semiconductor field effect transistor (hereinafter referred to as a “MISFET”) or a metal semiconductor field effect transistor (hereinafter referred to as a “MESFET”). A metal oxide semiconductor field effect transistor (hereinafter referred to as a “MOSFET”) is a kind of the MISFETs.
Such a switching element can be switched, by means of a voltage applied between its gate electrode and source electrode, between the on state in which a drain current of several amperes or more flows and the off state in which no drain current flows. In the off state, the switching element can withstand a high voltage of several hundred volts or more.
Further, a Schottky diode and a pn diode are typically used as rectifier elements, for example. These diodes are expected to serve as rectifier elements capable of withstanding a large current and a high voltage.
Since SiC has a dielectric breakdown field and a thermal conductivity which are greater than those of Si, designing a power device including SiC (a SiC power device) capable of withstanding a high voltage with low loss is easier than designing a Si power device capable of withstanding a high voltage with low loss. Accordingly, it is possible to produce a SiC power device which performs as well as a Si power device, and has considerably reduced area and thickness as compared to those of the Si power device.
Increasing the integration density of a power device such as a MISFET is effective at enabling a larger current to flow through the power device. In view of this, vertical power MISFETs with a trench gate structure have been proposed as a replacement for devices having a conventional planar gate structure. Since a MISFET with a trench gate structure includes a channel region formed on the side faces of a trench formed in a semiconductor layer, the unit cell area can be reduced and the integration density of the device can be increased.
A conventional semiconductor device which is a vertical MOSFET having a trench gate structure will be described below.
The conventional semiconductor device includes a substrate made of silicon carbide, a silicon carbide layer including an N-type drift region and a P-type body region and formed on the substrate, and an N-type source region formed in a portion of a surface of the body region. The conventional semiconductor device further includes a trench penetrating the source region and the body region and reaching the drift region, a gate insulating film covering the side faces and the bottom of the trench, and a gate electrode occupying the inside of the trench and located on the gate insulating film. A source electrode being in contact with the source region and the body region is provided on the silicon carbide layer, and a drain electrode is provided on the back face of the substrate.
In the vertical MOSFET thus configured, when a high voltage is applied between the source and drain, concentration of electric field is likely to occur on the bottom of the trench, which causes a dielectric breakdown in the gate insulating film on the bottom of the trench. To address this problem, a method of reducing an electric field applied to the bottom of a trench by forming a P-type region on the bottom of the trench has been proposed. For example, after forming a P-type region in a silicon carbide layer by ion implantation, a trench is formed (see Patent Document 1).